1. Field of the Invention
The present invention relates to buffer apparatus for storing carriers for holding wafers before and after input and output from a wafer processing area, and more specifically to a buffer incorporating a self teaching robotic system for moving the wafer carriers in the buffer.
2. Description of the Prior Art
In fabricating semiconductors, silicon wafers need to be stored in buffer facilities at various stages, such as before and after process steps. Due to the need for clean room standards in the buffer facility, the wafer carriers, boxes, pods or boats--collectively called carriers are preferably moved with robots in order to avoid possible contamination and to achieve the efficiency of automation. Each carrier must be picked up from an input position, placed on a shelf in a storage location, and then later moved to a platform for transfer of wafers into a processing area. The coordinates of the shelf or platform must be precisely known in order for processing robotics to be conveniently programmed to safely move the carrier. Due to the large dimensions of a buffer/storage area, it is not practical to hold either the absolute or relative carrier locations to the tolerances required by the robot. Neither is it usually practical to hold the robot tolerances required for accurate picking and placing
In order to assure the close tolerances required, the controller of the robotic element must be reprogrammed or "re-taught" new location data whenever a component is changed, or upon initial setup or when restarted. The term "teach" or "teaching" will be used to describe the process of gathering and entering component/structural location data into the system controller. Due to the need to minimize contaminants in the semiconductor processing environment, most robotic systems are installed in enclosures for control of the atmosphere. In prior art systems, it is generally necessary for a technician to enter the enclosure to position the robot while performing the teaching/calibration operations. These entries can contaminate the clean enclosure. In addition, the cramped, confined enclosure with moving robot parts presents a significant safety problem for the technician. This manual and awkward process is also time consuming and costly, and an inherently subjective process that relies upon the judgment and skill of the technician. For example, using conventional controls, a robot is installed and taught by jogging the robot around and, at each process station, the wafer placement locations are recorded with a teach pendant. Besides consuming many hours, this manual procedure introduces subjectivity and thus a significant possibility for errors. This creates a problem of reproducibility. Whenever a wafer carrier is not perfectly positioned within specification or a machine component wears, settles or malfunctions and requires replacement, the robot must be re-taught because it cannot automatically adapt to such variations. If the robot is not re-taught properly within close tolerances, serious damage or loss of expensive wafers can result.
It is clear from the above description of the prior art that an improved system for handling wafer carriers is needed to eliminate the requirement of an operator entering the buffer enclosure environment for calibration/teaching operations.